1. A REVIEW OF NITROUS OXIDE EMISSIONS FROM CROP PRODUCTION SYSTEMS, THE EFFECTS OF ADOPTION OF CONSERVATION TILLAGE, AND METHODS FOR THE REDUCTION OF NITROUS OXIDE EMISSIONS.
Owen Coxworth, Consultant, Saskatoon, Sask., S7H OJ3.June, 1998.
The use of conservation tillage (also called no till or zero till) has been shown to have a number of agronomic benefits. Soil erosion was reduced, moisture was used more efficiently, yields were increased in some cases, legume crops fixed more nitrogen, and the consumption of fuel and the use of machinery were both reduced considerably. Some of these benefits led to reductions in the emission of the greenhouse gas (GHG) carbon dioxide (less fuel and machinery used than in conventional tillage systems). Zero tillage also frequently increased the potential to store (sequester) more carbon dioxide in the soil as an increase in soil organic matter.
However, some early studies indicated that nitrous oxide emissions from the soil could increase with zero tillage. Nitrous oxide is a very potent GHG, with about 310 times the global warming potential (molecule per molecule) of carbon dioxide. Thus the concern was raised that even small increases in emissions of nitrous oxide from the adoption of zero tillage might cancel out the benefits of zero till, in terms of reductions in carbon dioxide emissions because of less fuel and machinery compared to conventional till, and in terms of increases in the sequestering of carbon in the soil. TranAlta asked us to determine the current status of knowledge about nitrous oxide emissions from zero tillage systems. This review would include determining how these nitrous oxide emissions from zero tillage systems compared with those from conventional tillage, and identifing gaps in knowledge requiring further experiments and studies. The literature search would include information on methods to reduce nitrous oxide emissions.
Nitric oxide (NO) can also be emitted from crop production systems, and is related to the amount of nitrogen fertilizer used. While it is not a greenhouse gas itself, production of nitric oxide can indirectly increase the concentration of greenhouse gases. Nitric oxide is a major factor in the development of air pollution in the lower part of the atmosphere. Our literature search was also directed to the question of how the adoption of zero tillage might affect levels of nitric oxide in the atmosphere.
An extensive literature review was conducted. In addition, various scientists, in western Canada and elsewhere, were interviewed by telephone and by personal contact. The main results of these investigations were as follows:
1. Effect of fertilizer nitrogen placement.
In one study in South Dakota, placement of nitrogen fertilizer in the surface layer of soil led to higher emissions of nitrous oxde from zero till compared to convention till. This result was similar to a number of earlier experiments in western Canada and elsewhere. However, when the nitrogen fertilizer was banded below this surface layer of soil, nitrous oxide emissions were much lower, in fact no higher than nitrous oxide emissions from conventional till. A common feature of many of the earlier investigations was that the nitrogen fertilizer was placed in the surface layer, rather than being banded at lower depths. No examples of measurement of nitrous oxide emissions from deep banded nitrogen (N) fertilizer have been found to date for western Canada. However, there is an extensive literature showing that deep banding of N fertilizer improves fertilizer use efficiency compared to surface incorporation. This means that a higher percentage of the band-applied N fertilizer is taken up by the plant and converted into grain and straw. This improved N fertilizer efficiency might be expected to reduce gaseous N losses, as by formation of nitrous oxide and its' loss into the atmosphere. This is consistent with the South Dakota results showing that deep banding reduced nitrous oxide emissions.
The early results which found higher levels of nitrous oxide emissions from zero till all seem to have employed surface application of N fertilizer. Many studies since then have shown that this placement of N fertilizer will tend to provide environmental conditions conducive to increased nitrous oxide emissions.
Experiments are recommended for western Canada to confirm that deep banding of N fertilizer, as is commonly used by actual farms employing zero till, reduces nitrous oxide emissions compared to surface incorporation. Deep banding with zero till should be compared to deep banding with conventional till and minimum till. One research group we talked to is considering such experiments.
2. Effect of including spring thaw nitrous oxide emissions.
Many previous studies in western Canada on nitrous oxide emissions from zero till systems (all with surface application of N fertilizer) measured only growing season emissions. In a recent study in Alberta, the effect of including spring thaw emissions was included. Zero till was compared to conventional till. Nitrogen fertilizer was incorporated in the surface layer of soil, not deep banded. In the spring thaw, nitrous oxide emissions were higher from the conventional till system. When spring thaw emissions were combined with growing season emissions, the conventional tillage system actually had higher total nitrous oxide emissions than the zero till system.We suggest that this type of experiment (full year emission measurements) be repeated with deep banding of N fertilizer, which would be expected to further reduce emissions from zero till systems.
3. Nitrous oxide emissions from legume crops used in zero tillage systems.
A recent review of nitrous oxide emissions from crops indicated that nitrous oxide emissions from legumes crops could be higher than emissions from cereals. Crop rotations including pulse crops and oilseeds are being recommended for zero tillage cropping systems and are being adopted by farmers in western Canada. Would an increase in the area planted to pulse crops lead to greater nitrous oxide emissions? We could not find any studies in western Canada which compared -nitrous oxide emissions from pulse crops such as lentils, peas or chick peas, with nitrous oxide emissions from cereals such as spring wheat. Experiments in Quebec found that soybeans, a major legume oilseed crop in eastern Canada, produced lower nitrous oxide emissions than corn, the major cereal crop grown in the eastern region. Furthermore, other experiments found that nitrogen fertilizer requirements for corn were reduced if the corn followed soybeans in the rotation. Would this reduce nitrous oxide emissions from the corn, assuming that less nitrogen fertilizer would lead to lower nitrous oxide emissions? Such measurements have not been done, as far as we can determine.
This leads us to recommend that similar experiments should be conducted in western Canada. These experiments would compare nitrous oxide emissions from pulse crops such as peas and lentils with nitrous oxide emissions from cereals such as wheat and barley. Studies should compare crops grown in a zero tillage system with a conventional tillage system, with nitrogen fertilizer banded on the cereals.
The effect of the pulse crop on the nitrous oxide emissions from a following cereal should also be measured, since it is known that nitrogen fertilizer requirements of wheat and barley are reduced if they follow a pulse crop in the rotation. What will be the effect on nitrous oxide emissions from the cereal?
4. Use of chemical additives or coating techniques to suppress nitrous oxide emissions from nitrogen fertilizers.
It is known that various chemical additives (e.g., nitrapyrin or coated calcium carbide) can suppress nitrous oxide emissions from nitrogen-fertilized crop production systems. Other studies have shown that coating nitrogen fertilizers with various materials, such as polyolefins, can slow the release of nitrogen nutrients so that they are more synchronous with the requirements of the growing crop. In some experiments in Japan, polyolefin coated nitrogen fertilizers were found to improve the nitrogen fertilizer efficiency, reduce the total amount of fertilizer required, increase yields and profits, and reduce nitrous oxide emissions.At least one Canadian fertilizer company (Agrium) is experimenting with coated fertilizers. An agronomist with the company reported to us that their main interest in such fertilizers was for applications in developing and/or tropical countries, where present use of nitrogen fertilizers is, in many cases, relatively inefficient (e.g., rice production). They believed that applications in western Canada were less likely, given the relatively good efficiency of nitrogen fertilizer use in this region (by such techniques as banding). One possible application could be in zero tillage systems, where coated nitrogen fertilizer might allow considerably higher rates of fertilizer to be applied close to the seed. This might reduce nitrous oxide emissions. The cost of coated fertilizers, relative to their benefits, will determine whether they find much use in western Canada.
It would be worthwhile to keep in touch with work on additives and coated fertilizers, since in some cases their use can lead to major reductions in nitrous oxide emissions. There may also be opportunities for GHG offset projects, particularly in tropical and semi-tropical countries, where there is expected to be a large increase in demand for nitrogen fertilizers in the future.
5. Effects of the type of nitrogen fertilizer on nitrous oxide emissions.
A recent Canadian study of the nation-wide emissions of greenhouse gases from agricultural activities used nitrous oxide emissions factors which indicated that nitrous oxide emissions from anhydrous ammonia and ammonium phosphate were from four to five times as high as those from urea or ammonium nitrate.
An earlier study in Iowa concluded that part of the reason for the high emissions from anhydrous ammonia arose from application as a concentrated band creating alkaline conditions in the soil band. One possibility to reduce these emissions might be to consider a wider, dispersed method of application to the soil. This was tested by the University of Saskatchewan, in cooperation with the Saskatchewan Wheat Pool. Nitrous oxide was measured by using chambers placed into the soil. This method gave very variable results, and no conclusions could be drawn from comparisons of deep, narrow banding with a more dispersed, wide band application. One possible analytical method which might resolve this sampling problem would be the tunable diode laser, which integrates nitrous oxide emissions over a broad area. Several institutes have such instruments, and it might be possible to arrange these type of anhydrous ammonia application method comparisons with them.
6. Effects of tillage systems on nitric oxide emissions.
We did not find to date any published studies which compared the effects of type of tillage system and nitrogen fertilizer placement method on nitric oxide emissions. Nitric oxide emissions tend to be highest at lower soil water-filled pore space contents than nitrous oxide (i.e., more aerobic soil conditions favor NO emissions). The ratio of N2O to NO varied widely between different studies.
We suggest that the same experiments recommended in items 1 to 5 include measurement of nitric oxide emissions as well as nitrous oxide emissions. The same experiments should serve to measure both N2O and NO.
7. Effects on nitrous oxide emissions of the reduction in the amount of summerfallow and increases in the amount of land in crop production.
It has been suggested that the adoption of zero tillage in western Canada, with the potential to store more moisture and use moisture more efficiently, has been a contributing factor in the substantial drop in hectares of summerfallow, increases in land in crop, and increases in nitrogen fertilizer usage in western Canada in the last decade. These changes in land use would be expected to reduce carbon dioxide emissions from soils, and increase carbon sequestering by means of an increase in soil organic matter.However, this reduction in amount of land in summerfallow, and an increase in land in crop, has been accompanied by an increase in use of nitrogen fertilizers. This increase in N fertilizer use would be expected to increase nitrous oxide emissions from soils, reducing the net GHG sequestering benefit of the increase in soil organic matter. The net effect needs to be measuredin future studies, which include nitrous oxide emission measurements.Furthermore, some studies have indicated that adoption of zero tillage may lead to an increase in nitrogen fertilizer requirements. This might lead to an increase in nitrous oxide emissions. Such studies on N2O emissions, comparing zero till with conventional till, have not been done, to our knowledge. It will be important in future experiments (see items 1 to 3) to measure changes in soil organic matter and related them to GHG emissions of nitrous oxide from soils and carbon dioxide emissions from inputs into crop production.
8. Overall conclusions.
The available evidence is quite limited, but does suggest that zero tillage is not likely to lead to an increase in nitrous oxide emissions, provided that the nitrogen fertilizer is banded, as is the common practice with farmers using zero tillage systems. Other new techniques of applying nitrogen fertilizers, such as nesting the nitrogen fertilizer, may in future also become common, and their nitrous oxide emissions will need to be measured. Full year emissions data from Alberta indicates that spring thaw nitrous oxide emissions from conventional tillage systems can actually exceed emissions from zero tillage systems.A series of experiments is suggested in items 1 to 3 which would confirm the effects of banding on nitrous oxide emissions and also measure emissions from pulse crops grown under zero Village systems. The same experiments could also measure nitric oxide emissions.Problems associated with the rapidly growing use of nitrogen fertilizers worldwide are becoming a major environmental issue. There may be real opportunities for more GHG offset projects, particularly in developing countries. These projects might not only reduce GHG emissions, but also help solve other environmental problems.
NOTES ON CONVERSATIONS HELD WITH VARIOUS SCIENTISTS ABOUT NITROGEN FERTILIZER PLACEMENT AND NITROUS OXIDE EMISSIONS.
1. Dr. Howard Woodard, Plant Science Department, South Dakota State University, Brookings, SD 57007. (605) 688-4774. June 10th, 1998.
Dr. Woodard conducted one of the very few studies located by database searches which looked at the effect of nitrogen (N) fertilizer placement on nitrous oxide emissions from zero tillage (J. Plant Nutrition 17 (8): 1341-1357 (1994)). This study at South Dakota State University found that deep placement of N in zero till reduced nitrous oxide (N2O) emissions to levels similar to soil incorporation of N fertilizer in conventional tillage, and much less than N2O emissions from zero till with surface application and incorporation of N. This study examined fertilizer placement during corn production in a corn-oat rotation. For deep placement, nitrogen (as a 28` N solution of urea-ammonium nitrate) was banded 10 to 15 cm below the soil surface using modified anhydrous ammonia applicator knives spaced 45 cm apart.
In my telephone conversation with him, I asked him if he was aware of other studies on the effects of N fertilizer placement on N2O emissions. He did not know of other studies. He stated several times that there was a big void in the literature on this subject. He stressed that more work is needed in this area. Their original work was not followed up by further studies, since B.R. Hilton, who did the work, moved on to another location.
In their original work, the N,Q emissions at the peak rainfall event were not obtained for deep placement of N fertilizer, because of a sampling error. However, N2O emissions from deep placement of N were obtained after other rainfall events, all indicating that deep placement reduced N2O emissions from zero till to levels as low as from N fertilizer incorporated in the surface layer in conventional tillage.
2. Dr. Reynald Lemke, Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, AB T1J 4B1. (403) 3274591, ext. 434. Two telephone conversations, one about June 11th, the second about June 15th.
In the first telephone conversation, Dr. Lemke told me that he had a draft paper describing his results on comparing full year N2O emissions from zero till and conventional till systems. He sent me a copy of this paper by courier. The paper has been submitted to the Can. J. Soil Sci.
In the paper, he reported that they (University of Alberta and Alberta Agriculture, Food and Rural Development) had compared the two tillage systems (zero till and conventional till) and hadincluded spring thaw emissions as well as growing season emissions. N2O emissions were higher from conventional till than zero till at spring thaw time. N2O emissions from zero till were slightly higher or similar to conventional till during the growing season. When the emissions from the two time periods were added up, the zero till had slightly lower total emissions than conventional till.However, the N fertilizer was surface applied in both tillage systems and incorporated in the conventional till system. Thus in the zero tillage system the N was not deep banded. When I talked to Reynald Lemke later (about June 15) on the telephone, he indicated that he was not aware of experiments in western Canada in which the effect of N fertilizer placement (such as banding) on N2O emissions had been done. He indicated that this was an important gap in our knowledge of N2O emissions and their management. There is an extensive literature on the improvement in N fertilizer use efficiency achieved by deep banding in zero tillage systems, but no studies on N2O emissions. He indicated that he and Henry Janzen were considering doing such a study.
3. Dr. Marie Boehm, Centre for Studies in Agriculture, Law and the Environment, University of Saskatchewan, Saskatoon, SK.
I talked to Dr. Boehm in person. She believed that studies on the effect of N fertilizer placement on N2O emissions had been done by Dr. Fran Walley, Soil Science Dept., Univ. of Sask., in cooperation with Garry Hnatowich, Saskatchewan Wheat Pool, Saskatoon, SK, at the Sask. Wheat Pool farm at Watrous, SK.
4. Garry Hnatowich, Saskatchewan Wheat Pool Agricultural Research and Development Branch, Saskatoon, SK (306) 668-6630.
I talked to Garry Hnatowich on the telephone. He reported that they had not measured N,O emissions or the effect of deep banding. He was not aware of this work having been done anywhere in western Canada. He suggested that the Watrous farm had two side-by-side fields, one in zero till, the other in conventional till, which could be.used for N20 experiments comparing deep banding under zero till with deep banding under conventional till. These fields have been in their respective tillage systems for a number of years. Thus the fields would be a good test of N2O emissions after a number of years in zero till, compared to conventional till. A simple paired T test would be possible at this site.
I talked to Garry in person on June 16th at his office in Saskatoon. He again stated that he was not aware of any studies in western Canada which compared the effects of deep banding and broadcasting in zero till and conventional till. We disussed the various comparisons which needed to be done (see diagram below). Of the various comparisons, the Wheat Pool farm at Watrous could do the comparison between zero till and conventional till with banding of fertilizers in both cases.
Model for studies:
Tillage system N fertilizer placement
Conventional tillage a b
Zero tillage c d
Comparisons: (a) compared to (b)
(a) compared to (c)(in the early literature)
(c) compared to (d)
(b) compared to (d)
I also asked Gary Hnatowich about anhydrous ammonia fertilizer and the reports that it could produce very high levels of N2O emissions compared to urea. Some of the effect seemed to be related to applying a very high concentration of anhydrous ammonia in a narrow band (results from Iowa, Breitenbeck and Bremner, 1986). What about applying ammonia in a wider dispersed pattern below the surface? This might also allow more placement of anhydrous ammonia closer to the seed at seeding time. He reported that Matus and van Kessel of the University of Saskatchewan had tried to do some measurements comparing dispersed banding with concentrated banding at the Saskatchewan Wheat Pool farm at Watrous. However, they ran into measurment problems using the canister method of collecting nitrous oxide samples.
I mentioned to him that the tunable diode laser might be a method around this problem. The University of Guelph has reported on using this method to measure N2O emissions from large field blocks with different treatments.Garry Hnatowich suggested that Richard Farrell and Diane Knight, a husband and wife team in the Soil Science Department of the University of Saskatchewan (replacing Chris van Kessel) might also be interested in N2O studies and the effect of fertilizer placement on emissions.
5. Dr. Jeff Schoenau, Soil Science Department, University of Saskatchewan, Saskatoon, SK, (306) 966-6844.
I talked to Dr. Schoenau by telephone on June 16th, 1998. He also reported that he was not aware of any studies in western Canada evaluating the effects of N fertilizer placement on N,O emissionsfrom zero tillage systems. He would be interested in participating in possible studies on this topic.
He has a site on his own farm near Central Butte (border of Brown and Dark Brown soil zones) where he has run chem fallow compared to tillage fallow for a number of years. He noted that in wet years, the available N after chem fallow was lower than after tillage fallow, suggesting that gaseous losses of N might be higher in chem fallow than in tillage fallow. This is in line with earlier studies by Dr. Don Rennie and his students.
In a separate phone call, Don Rennie reported to me that he believed that N2O emissions from banded fertilizer in zero till would be low, in line with the higher N use efficiency of such placement.
6. Dr. Bob Zentner, Semi-Arid Agriculture Research Centre (SPARC), Agriculture and Agri-Food Canada, Swift Current, SK (306) 778-7265.
I talked to Bob Zentner of the telephone on June 16th. I told him of my findings and asked if anyone at Swift Current was doing such studies on N fertilizer placement in zero till and N2O emissions. He told me that no-one was doing such studies. He thought that Brian McConkey would be interested. I indicated that the other gap in our knowledge of N2O emissions from cropping
systems was in the comparison of N2O emissions from pulse crops
(field peas, lentils, chickpeas) with wheat in zero tillage
systems (or in conventional tillage systems). Some studies have
reported that legumes can have very large N2O emissions under
some circumstances (e.g., plowing in a stand of alfalfa, or
soybeans planted after a fallow including manure incorporation).
In our discussions we agreed that measurements at a number of sites would be desirable. Examples we mentionned were:
1. Swift Current (Brown soil zone). Brian McConkey.
2. Lethbridge (Dark Brown soil zone). Reynald Lemke and Henry
3. Winnipeg (Moist Black soil zone). Martin Entz and David
4. Saskatoon (Dark Brown/thin Black soil zones). Jeff Schoenau
and possibly others.
5. Watrous (Dark Brown/thin Black). Garry Hnatowich, Sask. Wheat
We might consider such N2O studies as a complementary study to the Opportunities for Energy Reduction in Prairie Agricultural Production Systems (a PERD contract).
7. I corresponded by E Mail with Dr. C. van Kessel at the Dept. of Agronomy and Range Science, University of California at Davis,
Fax: (530) 752-4361, E Mail: email@example.com
He does not have further publications forthcoming about nitrous oxide emissions in western Canada except for one publication with Dan Pennock to be published in Biogeochemistry. He cautioned on assuming that improving the efficiency of N fertilizer would automatically reduce nitrous oxide emissions. There could be circumstances in which this was not the case. Thus experiments are needed to measure nitrous oxide emissions even when data is available on N fertilizer efficiency for various N fertilizer sources and methods of placement.
8. Dr. Marife Corre, United States Department of Agriculture, Agricultural Research Service, U.S. Pasture Laboratory, University Park, PA, U.S.A. E Mail: firstname.lastname@example.org
Dr. Corre reported to me that she and Dr. Dan Pennock of the University of Saskatchewan had studied the landscape effects of deep banding of urea nitrogen fertilizer in fertilizer cropland compared to pasture and forest. She and Dr. Pennock are in the proicess of writing up their studies in Saskatchewan. Nitr50us oxide emissions were compared amongst five different rates of urea fertilizer. Emissions were compared at shoulder, midslope, footslope and level depressional areas in the field. These studies did not compare deep banding with broadcasting fertilizer, or compare zero tillage with conventional tillage.Emissions of N2O were proportional to the amount of N fertilizer applied. emissions were highest in areas where high water-filled pore spaces were most likely to occur, such as footslpoes and level depressional positions.